Flame treating is used to improve the wetting and adhesion properties of polymer film surfaces in general and of polyolefin film surfaces in particular. The most wettable surface-modified polymer films usually have optimal adhesion properties in a variety of practical applications. These enhanced wetting properties result in improved coatability and adhesion of materials such as pressure-sensitive adhesives, primers and low-adhesion release coatings. Enhanced wetting properties are particularly useful in coating waterborne solutions at all film speeds and in coating solvent-borne materials at high coating speeds.
Flame treaters ordinarily use premixed flames, i.e., the fuel and oxidizer are thoroughly mixed prior to combustion and the rate of combustion is controlled by the rate of chemical reaction that occurs in the flame. In a premixed flame, the luminous region is that portion of the flame where the temperature rise is the greatest and where much of the reaction and heat release occur. During a flame-treating process, one side of a polymer film is passed in close proximity to a flame while the other side of the polymer surface generally passes over a cooled support, e.g., a cooled drum, to minimize heat distortion.
Flames are commonly described in terms of two characteristics: the flame power and the molar ratio of oxidizer to fuel. The flame power is the product of the volume of fuel burned per unit time and the heat content of the fuel. Typical units for the flame power are W or Btu/hr. In flame treating, the flame power can be normalized to account for the dimensions of the burner, leading to units such as W/cm.sup.2 or Btu/hr-in.sup.2. The exact ratio of oxidizer to fuel needed for complete combustion is known as the stoichiometric ratio. For example, the exact amount of dry air necessary for the complete combustion of methane is 9.55 volumes per volume of methane; so the stoichiometric ratio for an air:methane flame is 9.55:1. The equivalence ratio is defined as the stoichiometric oxidizer:fuel ratio divided by the actual oxidizer:fuel ratio. For fuel-lean, or oxidizing, flames, there is more than the stoichiometric amount of oxidizer and so the equivalence ratio is less than 1:1. For oxidizer:fuel mixtures at the stoichiometric ratio, the equivalence ratio is equal to 1:1. For fuel-rich systems, the equivalence ratio is greater than 1:1.
Virtually all industrial flame treaters use a premixed laminar (as opposed to turbulent) flame with air as the oxidizer and a gaseous hydrocarbon as a fuel. Typical hydrocarbon fuels include hydrogen, natural gas, methane, ethane, propane, butane, ethylene, liquefied petroleum gas, acetylene, or blends thereof, and city gas, which is often composed of a mixture of carbon dioxide, carbon monoxide, hydrogen, methane, and nitrogen. While carbon monoxide is also listed as a fuel in one patent, carbon monoxide does not burn in a steady flame with dry air as an oxidizer. Halogen and halogen-containing compounds have also been disclosed as oxidizer:fuel mixture additives to increase the adhesivity of polyolefin films to subsequent coatings.
Recently, hydrocarbon flames enriched with up to 5 percent additional oxygen by volume, up to 5 percent steam by weight and a few parts per million of alkali or alkaline earth metals have demonstrated increases in wetting values on polymer films (American Standard Test Methods (ASTM) standard wetting test) of up to 2 mJ/m.sup.2 relative to a non-enriched flame process. Surface modification of a polymer surface has also been reported by flame treatment where a flammable third component that is neither a fuel nor an oxidizer is sprayed into the flame. The listed third components are polymers such as cellulose, protein, silicones or polyethers, and inorganic materials such as carbides, nitrides, metal salts or metal oxides. While carbon monoxide plasmas and coronas are well known for improving the wettability of polymer films, plasma and corona processes are limited by their tendency to perforate thin or porous films, to "strike through" or inadvertently treat the backside of polymer films, and to create toxic or corrosive gases.
Because of the benefit that the increased wettability of polymer surfaces has on the coating industry, there is an ongoing need for processes that improve the wettability of polymer films.